1. Field of the Invention
The invention concerns an effective and reliable transmission technique with a scheme for retransmission of a lost packet in correlated fading channels.
2. Prior Art
At present, Internet and mobile communication technology has development with a growing trend to converge them, thus requiring mobile communication service, originally providing voice transmission service only, to also provide data transmission service. Because of correlated fading characteristic of a wireless channel, data packets can be lost, so there is a problem of unreliable transmission. Reliable link layer protocols, such as Automatic Repeat Request (ARQ), are one method to provide reliable transmission in an unreliable transmission system. ARQ methods can be roughly classified into Stop-and-Wait (SW), Go-back-N (GN) and Selective Repeat (SR). Among them, SR-ARQ is the most efficient and has been widely used in practical mobile systems, for example IS-99 (TIA/EIA/IS-99, “Data Services Option Standard for Wideband Spread Spectrum Digital Cellular System”, 1955).
FIG. 1 shows a working procedure of Selective Repeat (SR-ARQ). When the receiver sends back the Acknowledge (ACK) or Negative Acknowledge (NACK), the transmitter determines whether a specific data packet (for example #1) is lost or not, and makes a selective retransmission. When the transmitter receives an ACK signal, it means the #1 packet has been received successfully, it is unnecessary to retransmit again. When the Transmitter receives a NACK signal, it means that the #1 packet has not been received successfully, it is a transmission failure, the packet is lost and it is necessary to retransmit. When there is retransmission, each time only one copy of the lost #1 packet is retransmitted. The figure shows that after two times retransmission of #1 packet copy, it is received successfully. This means that in the traditional SR-ARQ scheme, when a packet is lost, only one copy of the lost packet is retransmitted each time. Obviously, the worse the environment of transmission, the more times retransmission is needed. In this case the data packet has a longer persistence time in the transmitter buffer and will seriously decrease the quality of data service.
Naturally, when a data packet is lost, multiple copies of the lost data packet can be retransmitted each time. But in a mobile communication system, because of the instinctive burst error characteristics in correlated fading channels data packets are successively lost and if multiple copies are sent for each retransmission they will meet the same bad state of the fading channels at the same time and the retransmission is failure, i.e. retransmission efficiency is low. Therefore, in correlated fading channels the issue of decreasing the number of retransmissions and increasing retransmission efficiency are two big problems needing consideration.